This study examined whether increased K supply in conjunction with BAP could increase lupin seed yield and harvest index by enlarging sink volume (pod number), increasing assimilate and improving assimilate partitioning to fill the additional pods induced by BAP treatment. Narrow-leafed lupin (Lupinus angustifolius, cv. Danja abs(-) mutant) was grown in a glasshouse, in pots containing sandy soil with four K treatments (0, 15, 60 and 120 mg K/kg soil). BAP (2 mM) was applied daily to all main stem flowers throughout the life of each flower from opening to senesced. BAP application did not affect assimilate production (as measured by total above-ground biomass), but changed assimilate partitioning. On BAP-treated plants, there were greater proportions of seed to pod wall dry weight on the main stem but smaller proportions on the branches, and an increased weight ratio of seed to pod wall overall which meant more assimilate was used for seed growth rather than pod wall growth. BAP increased the number of pods per plant by 35% and this more than compensated for the decreases in seeds per pod and seed weight. Therefore, there was an increased harvest index (+11%) and seed yield per plant (+13%) in BAP-treated plants. BAP also increased the number of pods with filled seeds (146%) on the main stem and main stem seed K+ concentration (from 0.81% to 0.87%). Added K increased biomass but only slightly affected assimilate partitioning. As applied K increased, relatively more assimilate was used for pod wall growth rather than seed growth. Added K increased seed yield per plant by about 14% due to increases in seed weight and the number of pods on the main stem. Moreover, K+ concentration in seeds and shoots increased with increasing level of applied K. Seed yield was enhanced more by BAP when K was supplied at high levels. Increasing K supply interacted positively with added BAP by increasing narrow-leaf lupin seed yield and harvest index through increases in assimilate supply and its partitioning into seeds.